The H.263 standard developed by the ITU (International Telecommunication Union) is a part of its H.324/H.323 recommendations for very low bit-rate multimedia telecommunication. The H.263 coding scheme which is described in "Video Coding for Very Low bit-rate Communication" Draft ITU-Recommendation H.263, may 1996, is based on earlier schemes used in H.261 and MPEG-1/2 standards, and using the Hybrid-DPCM concept comprising a motion estimation/compensation mechanism, and transform coding and quantization. Each image is divided into blocks of size 16.times.16 pixels (called macroblocks) and the macroblock in the current picture is predicted from the previous picture using motion estimation techniques. After the prediction, the macroblock is divided into four blocks of size 8.times.8 pixels. The prediction error is then transformed using the Discrete Cosine Transform (DCT) and the resulted coefficients are quantized and stored in the bitstream along with the motion parameters and other side information. The H.263 standard contains several improvements compared to earlier standards which allow a substantial reduction in the bit-rate while maintaining the same image quality. These improvements make it most suitable for very low bit-rate communication (but do not exclude it from being used in high bit-rate compression as well).
The H.263 coder supports several image sizes and does not have limitations on the bit-rate. These features allow the usage of H.263 in wide range of bandwidths such as the ones available in the Internet. ISDN, LAN, etc.
The H.263 includes the ability to indicate a block as &lt;not coded&gt; but the decision on whether a block is to be coded or not is not a part of the standard.
Real-life images very often contain information with a random nature (texture). When image sequences contain such static textures, the human observer rarely sees any difference in those textured parts when the video runs. If the image differences arm examined however, it is seen that large differences do exist. This is mainly due to the impact of imperfect sampling processes, microscopic camera movements, etc. The H.263 coder (as other standard coders like H.261 or MPEG1/2) is based on prediction and coding of the difference between predicted an original images. Textured static areas may therefore cause the encoder to spend many bits on these differences although no significant information should be coded there.